Field of the Invention
This invention relates to color cathode ray picture tubes, and is addressed specifically to an improved front assembly for color tubes having shadow masks of the tension foil type in association with a substantially flat faceplate. The invention is useful in color tubes of various types, including those used in home entertainment television receivers, and in medium-resolution and high-resolution tubes intended for color monitors.
The use of the foil-type flat tension mask and flat faceplate provides many benefits in comparison to the conventional domed shadow mask and correlatively curved faceplate. Chief among these is a greater power-handling capability which makes possible as much as a three-fold increase in brightness. The conventional curved shadow mask, which is not under tension, tends to "dome" in picture areas of high brightness where the intensity of the electron beam bombardment is greatest. Color impurities result as the mask moves closer to the faceplate, and as the beam-passing apertures move out of registration with their associated phosphor elements on the faceplate.
The tension mask, when heated, distorts in a manner quite different from that of the conventional mask. If the entire mask is heated uniformly, there is no doming and no distortion until tension is completely lost; just before that point, wrinkling may occur in the corners. If only portions of the mask are heated, those portions expand and the unheated portions contract, resulting in displacements within the plane of the mask; i.e., the mask remains flat.
The tension foil shadow mask is a part of the cathode ray tube front assembly, and is located in close adjacency to the faceplate. The front assembly comprises the faceplate with its screen consisting of deposits of light-emitting phosphors, a shadow mask, and support means for the mask. As used herein, the term "shadow mask" means an apertured metallic foil which may, by way of example, be about 0.001 inch thick, or less. The mask must be supported in high tension a predetermined distance from the inner surface of the cathode ray tube faceplate; this distance is known as the "Q-distance." As is well known in the art, the shadow mask acts as a color-selection electrode, or parallax barrier, which ensures that each of the three beams generated by the electron gun located in the neck of the tube lands only on its assigned phosphor deposits.
The requirements for a support means for a foil shadow mask are stringent. As has been noted, the foil shadow mask is normally mounted under high tension; e.g., 30 lb./inch. The support means must be of high rigidity so the mask is held immovable; an inward movement of the mask of as little as 0.0002 inch in the finished tube can cause the loss of guard band. Also, it is desirable that the shadow mask support means be of such configuration and material composition as to be compatible with the means to which it is attached. As an example, if the support means is attached to glass, such as the glass of the inner surface of the faceplate, the support means must have a coefficient of thermal contraction compatible with that of the glass, and by its composition, be bondable to glass. Also, the support means must be of such composition and structure that the mask can be secured to it by production-worthy techniques such as high-energy beam welding. Further, it is essential that the support means provide a suitable surface for mounting and securing the mask. The material of which the support structure is composed must be adaptable to machining or to other forms of shaping so the structure can be contoured into near-perfect flatness. Otherwise, voids will exist between the metal of the mask and the support structure, preventing positive, uniform contact of the mask to the support structure necessary for proper mask securement.
Means for securing the shadow mask support to the inner surface of the faceplate may comprise a cement in the form of a devitrifying solder glass, also known as "frit."
During the manufacturing process, a high-energy beam, noted as being a pulsed laser beam, is used to weld foil masks to the metal support structures secured to the inner surface of the faceplates. The same beam, in a higher-energy continuous-wave mode, is used to trim post-weld waste mask material. It was found that the overshoot of the cut-off beam, when falling on the glass of the inner surface of the faceplate, caused cracking and spalling of the glass. To prevent this destruction effect, an aluminum faceplate shield about 5 mils thick was initially used in production to intercept the high-energy beam and thus shield the glass of the faceplate. The shield was in the form of a frame which enclosed the mask support structure. The limited space between the mask and the panel made it difficult to design a structurally sound shield, one that would fit tightly enough against the support structure to prevent penetration of the cut-off beam past the shield. Also, the use of the shield required two additional process steps--that of emplacing the shield, and that of removing it. If by oversight, the shield was not put in place, a finished front assembly--that is, a screened assembly with the mask secured and ready to be fitted to a funnel, had to be scrapped because of consequent damage to the faceplate.